As the casing of a steam turbine or a gas turbine, 12 Cr ferritic steel having low thermal expansion coefficient compared with Ni-based alloys has been mainly used.
However, in recent years, for the improvement of the thermal efficiency, for example, a development has been pursued so that the steam temperature is increased to 650° C. or more in a steam turbine.
As the steam temperature thus becomes higher, the heat-resisting strength required of the casing also increases accordingly. However, for such a casing, it is possible for example to meet the requirement by increasing its thickness.
As the joint bolt for joining the casing, 12 Cr ferritic steel has been used as in the case of the casing. In the case of the joint bolt of the casing, the bolt can meet the requirement by increasing in size with an increase in temperature. However, this approach has a limitation, which necessitates the use of the one having a high heat-resisting strength at a higher temperature in terms of the material.
Examples of the materials therefore include austenitic Ni-base superalloys (e.g., Refractaloy 26 (trade name of Westinghouse So.)) having more excellent corrosion resistance and oxidation resistance, and higher high-temperature strength than those of the 12 Cr ferritic steels.
However, these have excellent high-temperature strength, but have a high thermal expansion coefficient. For this reason, the difference in thermal expansion from the casing of 12 Cr ferritic steels causes loosening of the bolt at high temperature, which may cause steam leakage.
The following references 1 and 2 each relate to a low thermal expansion Ni-base superalloy developed from such a viewpoint.
The Ni-base superalloy has been developed with the aim of making a superalloy having a thermal expansion coefficient close to that of the 12 Cr ferritic steel while keeping the high-temperature strength.    [Reference 1] JP 2003-13161 A    [Reference 2] JP 2000-256770 A
The present invention has been completed for the purpose of providing a method for producing a low thermal expansion Ni-base superalloy which has been further improved in creep fracture strength than the low thermal expansion Ni-base superalloys in the references 1 and 2, and which has a higher creep fracture strength under a high temperature atmosphere that is required for the joint bolt of a steam turbine etc.